Add documentation around inputs.d to config file (#2574)

The inputs.d feature was added in elastic/beats#30087 for elastic agent but unfortunately without any documentation. inputs.d directory is especially useful in the elastic-agent standalone mode. The close the first gap around documentation, I added it to the elastic-agent.yml file.

For awareness, this is an open issue around inputs.d directory: #622

Author: ruflin

_meta/config/common.p2.yml.tmpl

@@ -9,6 +9,11 @@ outputs:
     # username: "elastic"
     # password: "changeme"
 
+
+
+# Here you can configure your list of inputs. You can either configure all the inputs as a list of arrays
+# or create an "inputs.d" directory containing your input configurations. 
+# See https://www.elastic.co/guide/en/fleet/current/elastic-agent-configuration.html for how to structure the "inputs.d" directory.
 inputs:
   - type: system/metrics
     # Each input must have a unique ID.

elastic-agent.yml

@@ -15,6 +15,11 @@ outputs:
     # username: "elastic"
     # password: "changeme"
 
+
+
+# Here you can configure your list of inputs. You can either configure all the inputs as a list of arrays
+# or create an "inputs.d" directory containing your input configurations. 
+# See https://www.elastic.co/guide/en/fleet/current/elastic-agent-configuration.html for how to structure the "inputs.d" directory.
 inputs:
   - type: system/metrics
     # Each input must have a unique ID.